Generally, there are two types of such paper tractors, one being a sprocket wheel type and another being a sprocket belt type. In these tractors, a plurality of sprockets formed on the outer periphery of a sprocket wheel or a sprocket belt are arranged to engage perforations formed in the sides of recording paper. The recording paper is carried by rotating the sprocket wheel or sprocket belt to thereby rotate the respective sprockets sequentially.
The sprocket wheel or sprocket belt is accommodated in the chassis provided with an openable paper hold cover. It is provided movable along its drive shaft or support shaft integrally with the chassis in accordance with the width of the recording paper. It can be fixed at a desired position by a fixing device provided on the chassis. Conventional fixing means is disclosed, for example, in U.S. Pat. No. 4,315,585 in which a protrusion having a circular arc-like cross section extends from a fitting hole for the support shaft 26 formed in the chassis. Fitted to the protrusion is a fixing lever 16 in which the inner peripheral surfaces 66 receive the protrusion 40 and the fixing lever and protrusion are arranged to be eccentric with each other relative to the support shaft. When the fixing lever is swung in one direction, the respective inner peripheral surfaces are pressed against and engaged with the support shaft to thereby fix the chassis at a desired position while when the fixing lever is swung in another direction the engagement is released for axial movement of the chassis along the support shaft.
According to the conventional fixing apparatus, the chassis and fixing lever have a complicated configuration, and the support shaft and many parts must be provided, so that the manufacturing takes much time, the cost is high and miniaturization and lightening are hindered.
It is therefore an object of this invention to simplify the structure, render manufacturing easy, reduce the manufacturing cost and achieve miniaturization and lightening.
This invention is characterized by a paper tractor comprising; a driven shaft rotated when paper is fed; a rotary member not rotatable relative to the driven shaft and fitted to and slidable along the driven shaft; a plurality of sprockets provided engageable with respective perforations in recording paper and fed sequentially in the direction of rotation of the rotary member as the rotary member rotates; a chassis holding the rotary member on its both sides and movable widthwise of the recording paper; a fixed member supporting the chassis slidably via an engaging portion formed at one end of the chassis and extending widthwise of the recording paper; and a locking lever provided swingable at one end of the chassis, wherein the locking lever has a locking cam engageable with part of the fixed member, the locking cam including a first cam face for restricting the movement of the chassis by engaging part of the fixed member when the locking lever is swung to a locking position, and a second cam face for allowing the chassis to move relative to the fixed member in virtually non-engaged relationship thereto when the locking lever is swung to its released position.
When the locking lever is swung to a locking position, the first cam face of the locking cam provided on the locking lever is engaged with part of the fixed member to restrict the movement of the chassis and when the locking lever is swung to a release position, the second cam face of the locking cam opposes the fixed member in non-engaged relationship thereto to thereby allow the chassis to move widthwise of the recording paper.